In the last two weeks we have received a number of questions regarding the cause of reduced soybean stands in eastern Nebraska. Similarly, the UNL Plant and Pest Diagnostic Clinic (PPDC) has received a number of plant samples for diagnosis of herbicide injury and/or seedling disease problems.

Preemergence PPO Herbicides for Soybeans

Three PPO herbicides are predominantly used for pre-emergence weed control in soybeans. These are

These active ingredients provide residual control of many important annual broadleaf weeds in soybeans. They also can be effective in preplant burndown situations — depending upon the rate and weed species — because of their postemergence activity. They can be applied early preplant to shortly after soybean planting; however, severe injury can occur if they're applied as soybean is cracking through the soil surface. (All labels of these products warn against applications at this point.)

From a weed management perspective, using these products in a diversified herbicide weed control program is beneficial and can help curb the progression of glyphosate-resistant weed development in Nebraska. However, if these products are not used properly, as with most herbicides, crop injury can occur. In very general terms, injury potential increases

as the use rate increases, and/or

as soil organic matter decreases, and/or

the closer the application is to planting.

Additionally, there is a variable injury response among soybean varieties, ranging from highly tolerant to fairly susceptible. In the future, informing your seed dealer of your planned herbicide program is an important step to avoid pairing a susceptible variety with a preemergence PPO-based soybean herbicide program.

The unfortunate reality is that prolonged cool and wet weather conditions throughout much of eastern Nebraska in May and the first part of June were ideal for crop response to these products. Frequent or heavy rains as the hypocotyl arch is cracking through the soil surface or shortly after the cotyledons emerge can cause excessive herbicide uptake and subsequent injury. Cool temperatures reduce the rate at which the plant can metabolize (breakdown) the herbicide.

Injury will commonly display as reddish to purplish to brownish spots leading to necrotic tissue on the cotyledons and possible whittling of the stem at or near the soil surface. The potential to see injury often remains through the first few vegetative stages. Rains can splash soil with herbicide onto the unifoliates and first trifoliates, causing necrotic spotting and a crinkled appearance to some of the early developing trifoliates. In some instances it is difficult to assign specific levels of stand reduction to suspected causes such as herbicide injury, a soil-borne seedling disease, or cultural problems such as improper planting depth and soil crusting/compaction. Tissue injury from an excessive concentration or uptake of a PPO herbicide could cause points of infection for seedling diseases, while lack of vigor from seedling diseases could reduce a plant's ability to metabolize the PPO herbicide fast enough to prevent injury. Therefore, in many cases it can be difficult to identify the main cause of the injury.

Reduced Stands Can Still Perform Relatively Well

The fortunate reality is that soybean yield potential can remain relatively high, even at reduced populations resulting from herbicide injury or seedling disease. Given the current calendar date, destroying and replanting thin soybean stands should be considered in only the most extreme circumstances. Populations of 70,000-90,000 plants/acre and above should be accepted and retained at this point in the growing season.

While often difficult, it is important to correctly identify the cause of the stand reduction before determining future management actions for the field.